Ever since the conductive properties of acetylene and its analogues were described in the late 1950's, these types of compounds have attracted much attention for their electroconducting properties. See Heinze et al, Organic Electrochemistry, Marcel Dekker, New York, 2001; Lange et al, Anal. Chim. Acta, 614:1 (2008); Cosnier, Anal. Lett., 40:1260 (2007); Guimard et al, Prog. Polymer Sci. 32:876 (2007); Geetha et al, Anal. Chim. Acta, 568:119 (2006); Mermilliod et al, J. Electrochem. Soc., 133:1073 (1986); Kaynak et al, J. Appl. Polymer Sci., 54:269 (1994); Cen et al, Biosensors & Bioelectronics, 18:363 (2003); Lopez-Crapez et al, Clin. Chem., 47:186 (2001); Jager et al, Science, 290:1540 (2000); and Pawliszyn, Solid-Phase Microextraction: Theory and Practice, Wiley VCH, New York (1997). The conductive properties of various electronically conducting polymers, such as polyners derived from acetylene, and polymers such as polyphenylene (PPh), polyphenylene sulfide (PPhS), polyphenylene vinylene (PPhV), polypyrrole (PPy), polythiophene and polyaniline (PANI), commonly referred to as intrinsically conducting polymers (ICPs), have been investigated throughout the years. Some of these materials have been found to exhibit excellent conductivities but poor stabilities and/or processabilities, whereas others have been found to be less conductive but stable. Polypyrrole and polyaniline are two of the most promising currently known conductive polymers due to their reasonably high conductivities, good stabilities in the oxidized state, and ease of processing.
There are two main groups of applications for these polymers:
Based on ConductivityBased on ElectroactivityElectrostatic materialsMolecular electronicsConducting adhesivesElectric displaysElectromagnetic shieldingChemical & Biochemical sensorsPrinted circuit boardsBatteries and supercapacitorsArtificial nerve tissueDrug release systemsAntistatic clothingOptical computersPiezoceramicsIon exchange membranesDiodes and transistorsElectromechanical actuatorsAircraft structures“Smart” structures and switches
Polypyrrole (PPy) is one of the currently preferred conductive polymers due to its high electrical conductivity, facile synthesis both in aqueous and organic media, and relatively good stability in its oxidized state. During polymerization, anions in the electrolyte solution become incorporated in the polymer film to maintain the charge balance. The presence of these so-called dopant ions greatly influences the properties of the film. It is generally conceived that both anions and cations as well as accompanying water can move in or out of the polymer film upon its oxidation and reduction. If a small anion with high mobility is incorporated into a polymer film as a dopant during the polymerization, it will be expelled when the polymer is reduced. By doping the film with large anions with low mobilities, one can reversibly absorb and desorb cations which then move to maintain the electroneutrality. Various biologically active entities, such as enzymes (see Foulds et al, Anal. Chem., 60:2473-2478 (1998); and Rajesh et al, Curr. Appl. Phys., 5:184-188 (2005)), antibodies for immunosensors (see Xiao et al, Biosensors and Bioelectronics, 22(12):3161-3166 (2007)), or metal complexing entities (see Fabre et al, Coord. Chem. Rev., 178-180; 1211-1250 (1998)), can likewise be incorporated into PPy films to enable highly specific molecular and/or ionic recognition and separation. This phenomenon lays the foundation for various applications of PPy films in, e.g., ion exchange membranes and separation.
The Kaner et al U.S. Pat. No. 6,265,615 describes the use of polyaniline films for separation of optically active isomers of amino acids and pharmaceutically active ingredients. PPy composite materials were also investigated for enantioselective separation of amino acids (see Pich et al, Polymer, 47(19):6536-6543 (2006)). Further, the properties of PPy were shown to be useful in various devices for gas, chemical vapor, or moisture detection (Collins et al, Synth. Met., 78:93-101 (1996); and van de Leur et al, Synthetic Metals, 102:1330-1331 (1999)). The applications of conductive polymer composites are numerous, as the cited publications demonstrate, and many more uses will certainly appear in the future.
Functionalization with PPy of various natural and artificial polymers has also been described, including wool (Johnston et al, J. Appl. Phys., 6:587-590 (2006)) and textiles (Wu et al, Synthetic Met., 155:698-701 (2005)). Cellulose has been demonstrated to exhibit high affinity for, e.g., PPy or PANI, and that cellulose fibers therefore can be coated with these materials (Johnston et al, Synthetic Met., 153:65-68 (2005)). Further, the inclusion of small amounts of microcrystalline cellulose (MCC) was found advantageous as it significantly improves the mechanical properties of conductive polymers which otherwise are brittle (van den Berg et al, J. Mater. Chem., 27:2746-2753 (2007)).
Various technologies have developed a need for lightweight materials, flexible materials, and inexpensively produced materials that can be used as electrodes, for example, in sensors, batteries, extraction processes and the like. There is likewise a strong demand for new inexpensive ion exchange techniques for the processing of solutions containing various biologically interesting species, both on the micro and macro scale. Many of the currently employed ion exchange and separation techniques are time-consuming and labor intensive as they require large volumes of eluents for effective operation. These eluents are often expensive and often contain toxic reagents which need to be handled with care and discarded after use in an environmentally friendly fashion. Therefore, rapid and effective separation techniques utilizing a minimum of eluents are highly interesting. A promising alternative is electrochemically controlled ion exchange or electrochemically controlled solid-phase extraction techniques (see Gbatu et al, Anal. Commun., 36:203 (1999); and Liljegren et al, Analyst, 127:591 (2002)) which utilize the ion exchange properties of electronically conductive polymers. The latter techniques have the advantage that the ion exchange properties of the materials can be controlled merely using an electrical potential, enabling absorption and desorption of ions and polar neutral species in a controlled way by simply changing the applied potential of the polymer or the redox potential of the solution in contact with the material. Ion intercalation in electrode materials composed of electronically conductive polymers in energy storage devices is governed by similar processes as those involved in the ion exchange discussed above, with the difference that in the energy storage applications, the process of charge transfer and ion exchange is utilized to power electrical appliances rather than to extract or separate chemical compounds. Non-metal, lightweight, flexible, environmentally friendly electrode materials are of particular interest in energy storage devices.
Previous designs of electrochemically controlled ion exchange and solid-phase microextraction devices have been based on the deposition of a PPy film on metal electrodes wherein the ion absorption capacity was generally manipulated by controlling the thickness of the film. See, for example, WO 89/11648 of Biosyn R Corp. It is well-known that conductive polymer films can be electrochemically synthesized on the surface of an electrode. As the surface area of the electrode materials commonly used have been relatively small (i.e. on the order of cm2), mainly due to the use of electrode materials commonly used in electrochemical applications, the capacity of the polymer films has been modified by altering the thickness of the film.
Alternatively, films have been polymerized based on a chemical process involving a chemical oxidizing agent. Whereas electrodes with thick PPy coatings easily can be manufactured, their functionality with respect to absorption and desorption of ions and polar neutral species will generally be improved only to a limited extent by increasing the thickness of the polymer coating. This is because the absorption and desorption are mass-transport limited processes and the transport of ions and polar species relatively quickly becomes limited to the outermost layer of the film (see Liljegren et al, Analyst, 127:591 (2002)), especially for large species.
Several attempts have been made in the past to produce energy storage devices consisting entirely of lightweight components (Song et al, Adv. Mater., 18:1764-1768 (2006)). Polypyrrole (PPy) and its composite materials have attracted much interest in this respect as promising materials for the development of energy storage devices (Grgur et al, Electrochim. Acta, 53:4627 (2008)). For instance, composites of PPy with lightweight graphite fibers were investigated to obtain useful electrode materials for supercapacitors (Park et al, J. Power Sources, 105:20 (2002)). According to Rüetschi, the determining factors for a successful battery system are the 3-E criteria: Energy-Economics-Environment (Beck et al, Electrochim. Acta, 45:2467 (2000); Ruetschi, J. Power Sources, 42:1 (1993)). While conductive polymers are environmentally friendly and cheaper than their metal counterpart electrode materials, the low specific capacity and small operating voltage range have so far been limiting for their widespread use in commercial all-polymer battery systems (Ramakrishnan, Resonance, 48-58 (1997)). Additionally, the short life-time of the electrodes during charge-discharge has further hindered their use as feasible electrodes for energy storage devices (Id.).
Accordingly, further developments in electronically conducting polymer (intrinsically conducting polymer) materials are desired in order to accommodate use of such materials in various applications.